1. Field of the Invention
The present invention relates to a surface light source device and a liquid crystal display unit.
2. Description of the Related Art
In a liquid crystal display unit, liquid crystal itself does not emit light. Thus, for example, a so-called direct-type surface light source device (backlight) that radiates light to the display area of a liquid crystal display unit is arranged on the back surface of the display area formed by a plurality of pixels. It should be noted that in a color liquid crystal display unit, one pixel is formed by three sub pixels including a red light-emitting sub pixel, a green light-emitting sub pixel, and a blue light-emitting sub pixel. By causing liquid crystal cells constituting each pixel or sub pixel to operate as a kind of optical shutter (light valve), that is, by controlling the light transmittance (aperture ratio) of each pixel or each sub pixel, the light transmittance of illumination light (for example, white light) emitted from the surface light source device is controlled, thus displaying an image.
In the related art, a surface light source device of a liquid crystal display unit assembly illuminates the entire display area with uniform and constant brightness. A surface light source device having a configuration different from that of the above-mentioned surface light source device is known, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2005-258403, in which the surface light source device includes a plurality of surface light source units, and the illuminance distribution in a plurality of display area units are changed.
Such a surface light source device is controlled on the basis of the method as described below. That is, the maximum luminance of the respective surface light source units constituting the surface light source device is given as Ymax, and the maximum value (specifically, for example, 100%) of the light transmittance (aperture ratio) of pixels in the display area unit is given as Ltmax. Further, the light transmittance (aperture ratio) of each pixel for obtaining a display luminance y0 for each pixel in the display area unit when the respective surface light source units constituting the surface light source device have the maximum luminance Ymax is given as Lt0. Then, in this case, the light source luminance Y0 of each of the surface light source units constituting the surface light source device may be controlled so as the satisfy the following relationship.Y0·Ltmax=Ymax·Lt0 The conceptual illustration of such control is given in FIGS. 11A and 11B. In this case, the light source luminance Y0 of the surface light source unit is changed for every frame (referred to as the image display frame for the convenience of description) in the image display of the liquid crystal display unit.
By means of the control of the surface light source device (often referred to as the division driving of the surface light source device) as described above, it is possible to achieve an increase in contrast ratio due to an increase in white level and a decrease in black level in the liquid crystal display unit. As a result, it is possible to achieve an improvement in the quality of image display, and a reduction in the power consumption of the surface light source device.
In the technique disclosed in Japanese Unexamined Patent Application Publication No. 2005-258403, four light emitting diodes (LEDs) are arranged in one surface light source unit (see paragraph [0071] and FIG. 18 of Japanese Unexamined Patent Application Publication No. 2005-258403). The arrangement of four LEDs disclosed in Japanese Unexamined Patent Application Publication No. 2005-258403 is schematically shown in FIG. 12A.
Further, a part of the arrangement of light emitting diodes in a surface light source device that illuminates the entire display area with uniform and constant brightness is exemplified in FIGS. 13A and 14A. One example of such arrangement is featured on pages 124 to 128 of Nikkei Electronics, Vol. 889, Dec. 20, 2004. Here, in the drawings, the red light-emitting diode is indicated by a symbol with a character “R” enclosed inside a circle, the green light-emitting diode is indicated by a symbol with a character “G” enclosed inside a circle, and the blue light-emitting diode is indicated by a symbol with a character “B” enclosed inside a circle.